Free space, point-to-point communication systems are used extensively in the communications field. A network of point-to-point microwave systems can carry messages across the country as part of the public switched telephone network. Despite strong competition from fiber optic based communications systems, microwave or other free space systems are often justified for shorter routes, when right-of-way for a cable system is not available, or when the high communications capacity of a fiber optic system is not needed.
Laser communication systems in particular have become increasingly popular to provide a free space communications link between two locations. Laser systems do not require extensive frequency coordination as do microwave systems in common frequency bands. Laser systems often are less expensive to install than conventional copper cable or fiber optic cable communications systems because physical installation of a cable is unnecessary. For example, a laser communication system may have application between two corporate locations in a campus environment. Each laser communication terminal may be positioned on a building rooftop or even positioned adjacent a window and aligned to operate between buildings. A communications link within a building may also be provided by a free space laser communication system. Modern office automation also typically generates large amounts of data that must often be communicated between different corporate locations. Accordingly, the demand for laser communications links is increasing.
Free space laser communication systems are considered stationary laser sources for regulatory purposes, and as such, must comply with regulatory limits established to protect the eyes of an accidental or unintended observer. An accidental observer may receive permanent damage from a high power laser beam without experiencing any pain which might forewarn the observer of the harmful exposure. In addition, the wavelengths used by laser systems are often invisible. Accordingly, standards have been put in place that establish safe limits for the power that may be transmitted by a stationary laser source, such as a laser communication terminal. This maximum permissible power limits the communication system's signal-to-noise ratio, bit rate, and/or separation distance. Accordingly, there is a great need for a free space laser communication system and method that complies with safety limits yet which has improved performance over existing systems.
Moving laser beams present less of a hazard than do stationary beams as required for a free space communication system. For example, laser scanning systems for reading bar codes produce a moving, or non-stationary, laser beam. A spinning holographic disk produces a series of facet pulses from the beam. If the facet pulses are not detected, it is assumed the holographic disk is not spinning and thus the laser beam is stationary. The holographic scanner then operates the laser at a low duty cycle until the facet pulses are again detected.
Optical fibers are often used in conjunction with high power lasers, such as for laser welding and cutting, or medical applications, and the art has developed approaches for enhancing the safety of these systems. For example, U.S. Pat. No. 4,449,043 to Husbands discloses a safety device for a high power fiber optic system which may present a hazard when an optical connector is unmated. The safety device includes a four-port optical coupler which transmits to a receiver a portion of the output power, as well as backscattered energy which is developed between the glass-to-air and air-to-glass interfaces between adjacent connectors. A comparison between the output power and the backscattered energy is used to disable the laser source when an unmated condition is detected.
U.S. Pat. No. 4,543,477 to Doi et al. discloses a safety device for a medical laser wherein reflected laser light is detected from the exit end surface of a fiber and a shutter is used to stop the laser if a breakage of the fiber is detected. Ortiz Jr., in U.S. Pat. No. 4,812,641, discloses a high power laser for material processing and includes respective photodetectors to sense the laser power exiting a power optical fiber and the laser injection power. The two power levels are compared to detect whether a break in the power transmitting fiber has occurred. U.S. Pat. No. 4,673,795, also to Ortiz Jr., discloses an interlock safety arrangement which includes an optical sensor connected to the controller for turning the laser off when the laser beam has turned on but laser energy does not reach a remote module, indicating a break in the high power transmitting optical fiber.
Other safety mechanism for high power laser systems are also known. For example, U.S. Pat. No. 4,663,520 to Tanaka et al. discloses a main shutter in the path of a laser, and a safety shutter also in the optical path for completely intercepting the laser beam in its closed position. Sensors are provided before the main shutter and the safety shutter. A detection circuit opens the safety shutter when both sensors indicate predetermined values.
It is also known to vary the intensity of a laser for other than safety reasons. For example, U.S. Pat. No. 4,879,459 to Negishi discloses an automatic power controller for a semiconductor laser used in laser printing which can stop the control process when the intensity equals a set value despite the presence of electrical noise from a corona discharge. The controller includes an optical detector in a feedback control loop to equalize the intensity of the laser to a predetermined value. U.S. Pat. No. 4,862,397 to Pryor discloses an optical detector array, such as for industrial inspection and machine guidance, that adjusts light output levels, for example, to ensure that received light levels remain above a desired value. U.S. Pat. No. 4,837,428 to Takagi et al. discloses a driving circuit for a laser diode which regulates the intensity of the laser output by a controllable input voltage from a photodetector rather than a fixed reference voltage.
As described above, the prior art is primarily directed to detecting an unsafe condition in the output laser beam from a portion of the output beam reflected back to a detector. This approach is simply not applicable to a free space laser communication system which includes two widely spaced-apart terminals. An accidental or unintended observer may cross the path of the beams between the terminals or a beam may be blocked and reflected to the observer. In either case, a hazardous laser exposure could result if the laser were operated above the existing limits established for stationary laser beams.